Carbon dioxide emissions in the US

The United States has attempted to reduce carbon dioxide emissions to fight climate change. The line graph below shows how much carbon dioxide (CO2) was emitted from the United States between 2009 and 2013.

1.Which year did the United States had the greatest amount of CO2 emissions?

2.Between which years did the amount of CO2 emission decrease about 0.1 billion tons?

3.In which two years did the United States have the same amount of CO2 emissions?